U.S. Pat. No. 6,812,500 discloses a light emitting device comprising a GaN-based, particularly, AlInGaN-based light emitting diode capable of emitting ultraviolet or blue light, and phosphors absorbing a portion of light emitted from the light emitting diode and emitting light with converted wavelengths, thereby implementing polychromatic light, e.g., white light. Since such a white light emitting device uses a single-wavelength light source as a light source, its structure is very simple as compared with a white light emitting device using a plurality of light sources for different wavelengths.
Examples of phosphors used in the white light emitting device include an YAG:Ce phosphor using Ce3+ as an activator, an orthosilicate phosphor represented by Sr2SiO4:Eu using Eu2+ as an activator, and a thiogallate phosphor such as CaGa2S4:Eu.
These phosphors are generally prepared in a powder form through a solid state reaction method, and high-purity raw materials and strict stoichiometric compositions are required to synthesize these phosphors. Particularly, heat treatment at a high temperature of 1300° C. or more is required to synthesize YAG:Ce. This raises costs of the phosphors, leading to increase in manufacturing costs of the white light emitting device.
Further, since each of these powdered phosphors has many traps therein, it is likely to cause non-radiative recombination. Such non-radiative recombination leads to light loss, resulting in considerable reduction of wavelength conversion efficiency.
An object of the present invention is to provide a light emitting device employing phosphors that can be easily prepared to have high purity.
Another object of the present invention is to provide a light emitting device employing phosphors capable of reducing light loss due to non-radiative recombination.
To achieve these objects of the present invention, a light emitting device according to an embodiment of the present invention comprises a light emitting diode for emitting light having a first wavelength with a main peak in an ultraviolet, blue or green wavelength range; and nanowire phosphors for converting at least a portion of light having the first wavelength emitted from the light emitting diode into light with a second wavelength longer than the first wavelength.
The nanowire phosphors can reduce the number of traps as compared with conventional powdered phosphors, resulting in reduction of light loss due to non-radiative recombination.
Here, the term “nanowire” means a structure having a length relatively larger than the diameter thereof and having a nano-scale diameter of less than 1 μm.
The nanowire phosphor may be a nanowire made of ZnO, ZnO doped with Ag, ZnO doped with Al, Ga, In and/or Li, ZnO:Cu,Ga, ZnS:Cu,Ga, ZnS1-xTex(0<x<1), CdS:Mn capped with ZnS, ZnSe, Zn2SiO4:Mn, (Ba, Sr, Ca)2SiO4:Eu, or a nitride expressed by a general formula AlxInyGa(1-x-y)N (0≦x<1, 0<y<1, 0<x+y≦1).
With proper selection of a composition ratio of the nanowire phosphor, the light having the 2 first wavelength can be converted into the light having the second wavelength in a range of visible light.
Meanwhile, the AlxInyGa(1-x-y)N nanowire phosphor may have a composition ratio varying in a longitudinal direction such that the light having the second wavelength has at least two main peaks. Accordingly, in addition to the light having the first wavelength, polychromatic light having two or more colors can be implemented using one kind of nanowire phosphor.
Meanwhile, the nanowire phosphors may be formed on a substrate using metal organic chemical vapor deposition (MOCVD), metal organic hydride vapor phase epitaxy (MOHVPE) or molecular beam epitaxy (MBE). There is no specific limitation on the substrate and the substrate may be, for example, a silicon (Si) substrate. Thereafter, the nanowire phosphors are separated from the substrate. Thus, the nanowire phosphors can be easily fabricated, resulting in reduction of manufacturing costs.
A resin such as epoxy or silicone may cover the light emitting diode. The nanowire phosphors may be dispersed within the resin.
Meanwhile, the nanowire phosphor comprises a core nanowire and a nanoshell covering the core nanowire. The nanoshell prevents non-radiative recombination from being produced on the surface of the core nanowire. To this end, the nanoshell is preferably made of a material with a bandgap larger than that of the core nanowire.
Since nanowire phosphors are employed in accordance with the present invention, a manufacturing process is simplified to reduce manufacturing costs, and light loss due to non-radiative recombination is reduced to improve the efficiency of a light emitting device.